The present invention relates generally to circuit protection and more specifically to an improved electrostatic discharge protection.
Input protection of circuits of the prior art are well known as including one or more devices providing a current path between the input pin and one or both voltage supplies to shunt destructive input signals. The signals were generally caused by electrostatic discharge. These input protection devices are generally used to protect the gates of complementary insulated gate field effect transistors. These devices have included diodes, zener diodes, lateral bipolar transistors, resistive networks, and auxiliary insulated gate field effect transistors.
When the input protection circuit includes lateral bipolar transistors built using a gate over the base region, the voltage for which it can provide protection is limited by the thickness of the gate oxide breakdown voltage. This results because the gates extend to and often beyond the collector region. Therefore, the gate collector breakdown limits its effective protection.
Thus, it is an object of the present invention to provide an improved bipolar lateral transistor structure for use in an input protection circuit.
Another object of the present invention is to provide a lateral bipolar transistor having an equivalent structure to the insulated gate field effect transistors which it is to protect without being limited to the dielectric breakdown of the gate insulator.
These and other objects of the invention are achieved by forming the lateral bipolar transistor having the emitter an collector regions spaced in a base region and a gate over the base region separated laterally from at least the collector region. The gate may also be separated from the emitter region. The base region may include a low impurity base region in which the emitters and collectors are formed as well as a high impurity surface base region extending from the emitter, collector or the emitter and collector regions to the gates. The emitter region may include a deep high impurity region spaced from the gate and a shallow impurity region extending from the deep emitter region to the gate. The gate structure includes a gate and lateral spacers, wherein the lateral spacers extend over the base portion which separates the gate from the emitters and collectors and the shallow emitter portions. The input protection circuit includes a complementary pair of the improved lateral bipolar transistors having their collector regions connected to the input terminal and their base and emitter regions connected together and to appropriate voltage source.
Method of manufacturing the unique lateral bipolar transistor includes forming first and second gates on a portion of an insulative layer on P and N base regions followed by forming insulative spacers extending laterally from the gate regions. N and P impurities are introduced into the P and N-type base regions respectively using the gate and spacers as a mask to form emitter and collector regions laterally spaced in said P and N base regions and laterally spaced from the gates. As a variation to this method, higher impurity surface base regions may be formed between the gate and the emitter or the emitter and collector regions by introducing P and N impurities in the P and N base regions, respectively after the gate has been formed but before the lateral spacers such that the gate acts as a mask to form the high impurity base regions. Similarly, the shallow emitter extensions are formed before formation of the lateral spacers. The P and N base regions may be laterally separated by insulative inserts and the inserts in combination with the gate and the spacers form impurity masks. Impurities may be introduced to form base contact regions adjacent to the emitters and laterally spaced from the area covered by the gate and spacers.